Lasers are presently used in a variety of applications ranging from medical procedures to micro-machining and photolithography. Lasers have gained popularity for these applications for a number of reasons. First, by varying the wavelength of the laser the absorption characteristics of the laser beam can be tailored to a specific purpose. Second, by altering the focal length and the focus of the beam, the depth and type of treatment can be varied. For example, the same laser beam can be used to either heat treat or cut a material, the differences due to the focus of the beam. Third, by controlling the intensity and pulse duration of the laser output, different levels of treatment can be easily achieved.
Recently the use of lasers has become commonplace in a variety of medical procedures including both ophthalmic and cosmetic treatments. For example, the field of corneal reshaping has been revolutionized through the use of ultraviolet (i.e., UV) lasers (e.g., photo-refractive keratectomy). In these operations a pulse of relatively short duration is desired, typically on the order of between 1 and 50 nanoseconds. In contrast to ophthalmic treatments, medical procedures requiring high tissue absorption (e.g., surgery, cosmetic skin rejuvenation and resurfacing, etc.) typically require a relatively long pulse duration, on the order of 100 to 1000 microseconds, with a wavelength of between 1 and 5 micrometers. Due to both the wavelength and the pulse duration of interest in this type of procedure, free running erbium lasers have been particularly useful.
U.S. Pat. No. 5,290,274 discloses a system that allows radiation of two different wavelengths to be simultaneously or individually applied to a given area. The two different wavelengths are produced by two different lasers, the individual wavelengths selected on the basis of absorption by a given organic tissue. The disclosed system also includes means for simultaneously directing a cooling fluid at the tissue being treated by the radiation sources. In the preferred embodiment of the disclosed system, the selected lasers are a Nd:YAG laser and an Er:YAG laser.
U.S. Pat. No. 5,655,547 discloses a technique of using multiple lasers in a surgical procedure. In this technique, the portion of tissue to be treated is simultaneously irradiated with multiple, coaxial laser beams. As disclosed, by selecting the wavelength of one laser such that the extinction length in the skin tissue is between 0.01 and 0.001 millimeters, and selecting the wavelength of a second laser such that the extinction length in the skin tissue is between 1 and 0.1 millimeters, the skin undergoes simultaneous ablation, coagulation, and shrinkage.
U.S. Pat. No. 4,791,927 discloses a technique of simultaneously irradiating a biological target with infrared radiation and ultraviolet radiation from a pair of lasers. The ultraviolet radiation cuts the target tissue while the infrared radiation cauterizes it. The use of alexandrite and xenon fluoride lasers is disclosed, as is the use of harmonic generators and Raman cells.
U.S. Pat. No. 5,144,630 discloses a multi-wavelength laser system utilizing a pulsed Nd:YAG or Nd:YLF laser. Various wavelengths are achieved through the use non-linear crystals. To achieve ultraviolet radiation, the output of the solid state laser is passed through a series of three non-linear crystals in order to obtain the fifth harmonic of the solid state laser. Infrared radiation is obtained from the solid state laser through the use of an optical parametric oscillation cavity. As the solid state laser is either Q-switched or mode locked, the pulse length of either the ultraviolet radiation or the infrared radiation ranges from subpicosecond to a few nanoseconds.
Although a variety of techniques have been developed for multi-wavelength laser systems, what is needed in the art is a laser system utilizing a single solid state laser and producing short pulses of ultraviolet radiation or long pulses of infrared radiation. The present invention provides such a system.